This competing renewal describes genetic experiments designed to elucidate the molecular basis of the plant defense response to bacterial pathogen attack. The biological system employed involves infection of the small crucifer Arabidopsis thaliana with the well-studied phytopathogenic bacterium Pseudomonas syringae. P. syringae strain ES4326 (Psm ES4326) elicits disease symptoms on Arabidopsis leaves, whereas the same strain carrying the cloned avirulence (avr) gene avrRpt2 elicits a so-called hypersensitive defense response (HR). We initially focused on the gene- for-gene interaction between avrRpt2 and the corresponding Arabidopsis resistance (R) gene RPS2, the founding member of a class of genes that confer resistance to viral, bacterial and fungal pathogens in a variety of plants. The first Aim of this competing renewal is to identify other components of the avrRpt-RPS2 signal transduction pathway by screening for Arabidopsis mutants that aberrantly express avrRpt2-induced genes. The second Aim is to extend the analysis of the Arabidopsis-P.syringae interaction to preliminary results indicate that Arabidopsis actively defends itself against Psm ES4326 not carrying avrRpt2, we screened for Arabidopsis mutants which exhibited enhanced disease susceptibility to Psm ES4326 and identified 26 "eds" mutants. In AIM 2, eds and related mutants will be sorted into complementation groups and mapped. In Aim 3, the defense-related genes identified in Aims 1 and 2 will be assigned to specific defense-related signal transduction pathways by monitoring their susceptibility to a panel of known Arabidopsis bacterial and fungal pathogens, by monitoring the timing and level of expression of a variety of defense related responses, by determining whether any of the mutants specifically compensate for the pathogenicity defects of a panel of previously isolated Psm ES4326 non-pathogenic mutants, and by constructing selected double mutants. In Aim 4, we will determine whether any of the genes identified in Aims 1 and 2 correspond to known pathogen-induced genes, we will screen T-DNA insertion libraries for insertions in known pathogen-induced genes, and we will inactivate selected pathogen-induced genes using anti-sense constructs. Finally, in Aim 5, we will clone and characterize selected Arabidopsis defense-related genes using a map-based positional cloning strategy.